1. Field of the Invention
This invention relates to the field of automotive exhaust components; and more particularly, to a muffler, catalytic converter or the like, that is formed and housed within a seamless enclosure.
2. Description of the Prior Art
It is widely recognized that the exhaust emissions of internal combustion engines constitute a major source of air pollution throughout the world. The combustion process in these engines inevitably results in the production of certain substances that pass into the exhaust stream and are detrimental to the health and well being of humans and other animal and plant species. The emissions of concern include particulates (soot) along with gases such as CO, SO2, NOx, and imperfectly burned hydrocarbons (HC). These substances are produced in the combustion process, along with the CO2 and H2O that are the products of the complete oxidation of the hydrocarbons comprised in fuel.
The combination of market forces and governmental environmental regulations has spurred research and development of ways to mitigate or eliminate the production of the harmful constituents in engine exhaust. Automakers and suppliers have been challenged to control and reduce vehicle tailpipe emissions by the U.S. Clean Air Act of 1965 and subsequent legislation in the U.S. and other countries. In response to this legislation, virtually every system in the engine has been improved. As a result, modern engines more efficiently convert the latent chemical energy in fuels to useful mechanical work, so that their emissions are markedly reduced.
To date significant efforts have been directed toward the four-stroke Otto engine in passenger automobiles, owing to consumer preferences and government action. Despite progress in emission reduction for these automobile engines, increasingly stringent limits are being imposed. Emission regulations have also extended to other on-road vehicles, such as busses, and trucks, many of which employ diesel engines; to off-road vehicles; and to non-propulsion engines, many of which are two-stroke.
Much of the reduction in noxious emissions is attributable to use of catalytic converters through which exhaust gas streams are directed. The passage of the exhaust across a surface comprising a suitable catalyst promotes further chemical reaction that removes a substantial fraction of the noxious CO, NOx, and HC substances, converting them instead into more benign substances such as CO2, O2, N2, and H2O. Moreover, use of catalytic converters in combination with computer-driven, adaptive control of timing and fuel-air mixture gives an engine designer significant flexibility when optimizing engine-operating parameters to achieve reduced emissions.
Notwithstanding the market pull coming from the significant advantages realized by interposition of catalytic converters in the exhaust stream, there remain substantial impediments to their manufacture. It would be desirable if converters could be manufactured using reliable, efficient and inexpensive construction processes; and maintained durability and functionality over a prolonged service life. However, conventional converters fail to afford these desirable characteristics.
Converter constructions must produce a gas-tight enclosure so that exhaust enters solely at an inlet port and exists exclusively through an outlet port. Failure to achieve a hermetic sealing deleteriously allows leakage of exhaust gas, circumventing the beneficial effect of the catalyst and producing unacceptable noise. In some cases, leakage of exhaust containing combustible gases can lead to engine backfiring and damage to other portions of the engine system. Leakage can also expose vehicle occupants to unhealthy or dangerous levels of CO and other emissions. In addition, leaks have been known to trigger catastrophic vehicle fires.
Understandably, automobile manufacturers are impelled by several factors to minimize or eliminate these catalytic converter failures. The reputation of a manufacturer as a supplier of a high-quality product is degraded by reported failures. In addition, both market forces and current U.S. environmental regulations compel an auto manufacturer to warranty the integrity and efficacy of all aspects of an auto's pollution control system. More specifically, the regulations require that the system function to maintain the auto's emissions within established standards for an extended period of time and mileage. Any failures expose the manufacturer to costly warranty repairs and to the ire of an inconvenienced consumer.
Heretofore, the metal housings used for catalytic converters have mostly fallen into three broad categories of construction: a “pancake” or “clamshell” form, a wrapped form, and a multipiece form, each of which encloses a catalytic substrate bearing catalytically active material.
Typically, the “pancake” or “clamshell” form comprises stamped upper and lower shells, which are substantially identical to each other, and which have mating, peripheral, side flanges that are welded together to lie in a plane containing the longitudinal axis of the housing. They are shaped to form an internal chamber in which the catalytic substrate is mounted by “L-shaped” or other known brackets or pre-formed features provided integrally in the housing component shells.
The wrapped-form housing is made with material that initially is sheet-like and formed so as to generally encircle the catalytic substrate. This form is also known as a “tourniquet wrap,” reflecting its construction. The edges of the housing must be joined at a welded seam that runs essentially the full axial length of the converter. The inlet and outlet ports in this construction may either be formed as part of the wrapping operation or, more commonly, may comprise separate components welded to the ends of the housing subsequent to the formation of the sheet material.
Several multipiece housing constructions are known. One form disclosed by U.S. Pat. No. 5,118,476 comprises a tubular middle section in which the catalytic substrate is placed and end bushings attached to each end of the middle section. U.S. Pat. No. 6,001,314 discloses a two-piece housing. Each of the pieces is shaped by deep drawing to provide an open end and a conical outer end tapered to an opening appointed for connection to associated exhaust system pipes. The two pieces are welded together with the catalytic substrate contained within.
Each of these multi-piece constructions must be sealed by welding, either to close a seam in a sheet-like material or to affix appropriate end caps. The welding is needed both to provide the required hermetic sealing of the housing and to secure the catalytic substrate. However, each of these welds is a likely failure mode. Moreover, the OBD2 (on-board diagnostics) standard mandated by the U.S. Environmental Protection Agency for passenger vehicles after 1996 imposes a further need for hermetic integrity in the engine exhaust system. This standard mandates measurement of O2 content before and after the catalytic converter as a required input for the computerized engine control system. Even a pinhole leak in the system between the sensors compromises the accuracy of the comparison in O2 levels which is used for a mass balance determination. The emissions control system fails, negating the ability of the engine control system to adaptively optimize timing and fuel/air mixture to minimize emissions. Such failure of the emissions control system must be corrected under warrantee by the vehicle manufacturer at considerable expense and inconvenience to the consumer.
The environment of a catalytic converter is harsh for a multiplicity of reasons, each of which can potentially cause penetrating corrosion and ultimate failure of the converter housing. For example, a converter used in an on-road vehicle, especially in cold climates, is exposed externally to a spray of road salt and internally to acidic exhaust gases. It is well known in the art that chemical and stress effects combine to make weldments especially likely loci of corrosive attack. Accordingly, a catalytic converter housing that could be formed efficiently and economically into a single piece without welding has long been sought in the automotive art. Such a one-piece catalytic converter housing would overcome serious shortcomings involving the reliability of extant multi-piece and welded housing forms.
The only known technique for producing single-piece housings is spinning. In this process, a catalytic element is placed within a tube and the combined workpieces are rapidly spun about the tube's cylindrical axis while suitable tools are brought into contact with the tube at each of its ends. Sufficient deformation is thus accomplished to form tubular ports of reduced diameter at each end of the tube. Depending on the required reduction, the spinning may be carried out either cold or hot. While the spinning approach does produce a single-piece housing, it also carries substantial drawbacks. The production forming is expensive and energy-intensive to conduct. Moreover, it results in formation of circumferential ridges on both the inside and outside surfaces of the housing in the deformed region. These ridges are both unattractive and present significant disruption of the gas flow inside the muffler, causing turbulence and undesirable back pressure that reduce the engine power available for a given cylinder displacement. The magnitude of diameter reduction achievable by spinning is limited. In addition, substantial work hardening is produced in the metal in the reduced section. As a result, the ductility of the tube in the reduced section, including the port tubulation, is too low to allow the converter to be attached to adjoining exhaust sections by ordinary clamps. Welded or flanged joints must be used instead. The need for welding joints is particularly inconvenient for aftermarket and repair use.
The spinning process is further limited by the size and shape of product that it can produce. Very long shapes are unwieldy to secure and spin at the required rate in available lathes and similar machine tools. Moreover devices produced by spinning must be rotationally symmetric about a cylindrical axis, or the resulting imbalance makes it impossible to spin the device and accomplish the needed forming of the desired shape. In many cases the circuitous path available for the exhaust system would make it highly desirable to have non-symmetric components available, such as a catalytic converter in which the inlet and outlet ports need not be coaxially aligned, but angulated or offset relative to one another. Such configurations cannot be formed by known spinning methods. Furthermore, areas of the housing formed by spinning are work-hardened to an extent that renders subsequent bending and like operations virtually impossible.
As a result of these deficiencies, spinning is not widely used in the manufacture of exhaust components, notwithstanding the eagerness of the market for a viable single-piece, seamless device which spinning might be thought capable of producing.